1. Field of the Invention
This invention relates to a method of toughening shaped articles composed of compacted polymeric particles wherein said polymer is a thermally stable, intractable, substantially linear aromatic polymer and to the toughened shaped articles obtained thereby.
2. Description of the Prior Art
Polymers having predominatly aromatic character have long been recognized as offering a major advantage in thermal stability over the more conventional aliphatic polymers. Polymers of benzene, especially poly(p-phenylene) have been particularly attractive because of their unusual resistance to thermal decomposition and their potential low cost. A high-yield method for polymerizing benzene developed by Kovacic (e.g., see Kovacic and Kyriakis, Jour. Am. Chem. Soc., 85, p. 454, 1963) had led to intensive activity to develop a commercial high temperature polymer based on poly(p-phenylene).
The infusibility of pure poly(p-phenylene) and its insolubility in conventional organic solvents has heretofore prevented the utilization of its good high temperature properties in shaped articles. The powdered polymer obtained by the Kovacic procedure has been pressed into relatively weak articles that have some compressive strength. Using isostatic pressing, tensile strengths as high as about 2000 psi have been obtained (G. K. Ostrum, D. D. Lawson, and J. P. Ingham, Polymer Preprints, I (2), p. 895, 1966). Attempts have been made to reduce the brittleness of poly(p-phenylene) shaped articles by hot pressing and by annealing of pressed compacts. Kovacic et al. (Jour. Appl. Poly. Sci., 12, pp. 1735-1743, 1968) show that maximum strength is obtained by pressing near room temperature and that subsequent annealing at temperatures up to 500.degree. C decreases fracture strength. The decrease in strength was found to correlate with increasing x-ray crystallinity and was attributed to brittleness that develops as amorphous polymer regions are crystallized. At 600.degree. C all crystallinity was lost as the polymer began to degrade by loss of hydrogen.
The intractability of polybenzenes has diverted further development efforts in two opposite directions. On the one hand, the compressed, but weak, articles of polybenzene have served as precursors to high-strength carbonaceous articles obtained by pyrolysis at temperatures ranging from 650.degree. to 2500.degree. C. Molding of poly(p-phenylene) and subsequent pyrolysis are taught in British Pat. No. 1,156,041 and related U.S. Pat. Nos. 3,443,899 and 3,498,929. The yield of char can be maximized by using compositions with substantial chlorine content so that heating eliminates HCl to produce a cross-linked structure that is more resistant to loss of carbon at high temperatures (Vincent, Jour. Macromol. Sci. -- Chem., A3(3), pp. 485-499, May 1969).
On the other hand, efforts have been made to chemically modify the desirable polyphenylene structure to reduce the rigidity of its polymeric backbone so that conventional forming techniques such as injection molding and solution casting can be applied. Modifications have included highly branched polyphenylenes, polymerization in the presence of oxygen and in nitroalkane solvents, the use of fused ring monomers which are taught in a series of patents to Bilow and coworkers (U.S. Pat. Nos. 3,555,108; 3,565,832; 3,578,611 and 3,582,498); and the use of nonaromatic linking groups such as amides as taught in Canadian Pat. No. 870,688. By their increased fusibility and solubility such polymers compromise the inherent advantages of the thermally stable aromatic compositions.